Reducing the dimensions of a gate dielectric of a semiconductor device, such as a metal oxide semiconductor (MOS) field effect transistor (FET), improves the performance of the device. For instance, a thinner gate dielectric is desirable because this increases the capacitance and the operating frequency of the transistor. Gate dielectrics are often made by thermally growing silicon dioxide (SiO2) to a thickness of about 20 Angstroms or less. Such ultra-thin gate oxides, however, encounter a number of problems, including dopant penetration from the electrode material and direct carrier tunneling. This, in turn, causes high leakage current and threshold voltage effects thereby degrading device performance and device reliability.
These problems can be minimized by nitridation of the gate oxide. Nitrogen incorporation allows for thicker gate oxides to be produced but with electrical properties that are equivalent to an ultra-thin gate oxide. The thicker gate inhibits or reduces the problems previously mentioned. The performance characteristics of transistors depend on both the concentration and distribution of nitrogen atoms incorporated into the gate dielectric. If the nitrogen content of the gate oxide changes, then the electrical characteristics of the transistor will also change. For instance, if the nitrogen content of the gate oxide decreases, then the gate oxide will have the unsuitable electrical characteristics of a thicker gate.
It is desirable to have the ability to monitor the nitridation process both between wafers and within wafers so as to manufacture devices having uniform properties. Consider, for example, a 200 mm diameter semiconductor wafer comprising about 100 to about 1000 integrated circuit chips. Preferably, each integrated circuit chip on the wafer has transistors whose performance characteristics are substantially identical. Furthermore, it is advantageous for chips constructed on different wafers to have substantially identical performance. It is also advantageous if the monitoring of the gate oxide nitridation process can be employed in or near a transistor production facility so that corrections to the nitridation process can be made rapidly during mass production. Previous procedures used to monitor the gate oxide nitridation process are problematic, however.
Previous methods to monitor gate oxide nitridation entailed quantifying the nitrogen content of the gate oxide. Examples include time-of-flight (ToF) secondary ion mass spectroscopy (SIMS), or X-ray photoelectron spectroscopy (XPS). A determination of the nitrogen content, however, is usually done on only one sample per wafer. In addition, these methods usually require significant amounts of production time to transport the gate oxide sample from the production facility to the analytical lab to have it analyzed. During this time period, even though the quality of nitridation is unknown, wafer lot production continues. Thus, wafers can be produced during this analytical time period that may not fall within desirable design parameters.
Other disadvantages associated with present processes stem from the fact that the rapid determination of changes in the nitrogen content of the gate oxides being fabricated in the production facility is not completely ideal. For instance, it is not optimally desirable to use the above-mentioned analytical methods to monitor and compensate for inadvertent drifts in the process used for nitridating gate oxides in a production environment due to the time factors mentioned above and the questionable impact on the quality of the nitrided gate oxide. It is also very difficult to monitor non-uniformities in the nitrogen content of the gate oxide formed across the surface of individual wafers in a production environment.
Accordingly, what is needed in the art is an improved method of monitoring the nitration of gate oxides in transistors that avoid the above-mentioned limitations.